1. Field of the Invention
This invention relates to a three-terminal filter using area flexural vibration mode for use in, for example, an AM filter or other suitable apparatus.
2. Description of the Related Art
Conventional three-terminal filters used in a kHz band include a filter using the area expansion vibration mode or a filter using the length vibration mode.
FIG. 1 illustrates an example of an AM three-terminal filter B using the area expansion vibration mode.
In this filter B, a center electrode 11 is provided on the surface of a central portion of a piezoelectric ceramic substrate 10 having a square shape, a ring electrode 12 that surrounds the center electrode 11 is provided on the exterior thereof, and a ground electrode 13 is provided on the entire back surface. An input terminal 11a, an output terminal 12a, and a ground terminal 13a are respectively connected to the center electrode 11, the ring electrode 12, and the ground electrode 13.
FIG. 2 shows a circuit diagram of the three-terminal filter B of FIG. 1.
In the three-terminal filter B using the area expansion vibration mode, the resonance frequency is determined by the length of one side of the three-terminal filter B. For example, if a 40 kHz filter is desired, the length of one side of the filter is 50 mm.
In recent years, miniaturization of electronic devices is increasingly important, and thus, electronic components are also required to be made smaller and thinner. The above-described filter is relatively large and cannot be sufficiently miniaturized. Therefore, the three-terminal filter using the area expansion vibration mode was applicable only for filters having a frequency of about several 100 kHz or more.
In a three-terminal filter using the length vibration mode, as with the filter using the area expansion vibration mode, since the resonance frequency is determined by the shape (length) thereof, size-reduction is very difficult.